In a semiconductor integrated circuit which has a multilayer wiring layer, in order to connect a certain wiring layer and another wiring layer mutually, via structure is used. Generally, in a signal wiring except a power supply wiring, connection between different wiring layers by using one via per location is made. Hereafter, such a general via is referred to a “single-cut via.”
With following miniaturization of semiconductor integrated circuits in recent years, wiring width has been reduced, and an area of a single-cut via has been also small. Hence, it has become hard at the time of a production process to form a single-cut via with a desired pattern. In a worst case, an open fault arises in a via-forming section, and a desired device operation is no longer achieved. This causes a drop of a yield. Furthermore, with following shrinkage of a single-cut via, a delay time in signal wiring becomes large, and a disconnection probability by electromigration also becomes high. These cause a drop of operational reliability of a device.
As a technique for suppressing such a malfunction, “redundant vias” are proposed as described, for example, in U.S. Pat. No. 5,798,937 and the like. According to the redundant via technique, in addition to an original single-cut via, a “redundant via” is provided. In further detail, redundant via software extracts a single-cut via from layout data after a detail routing step. Then, the redundant via software places a redundant via in parallel around the extracted single-cut via. Then, design rules are checked. When there is no violation, the redundant via is adopted, and in the case of a violation, the redundant via is eliminated. In this way, a redundant via is added to at least a part of a signal wiring of a design object circuit. In this case, even if a defect arises in any one of the original via and redundant via, electrical connection of the signal wiring is secured. In consequence, an event probability of the above-described malfunction is reduced.
The followings are known as latest techniques relevant to formation of a redundant via.
According to a technique described in Japanese Patent Laid-Open No. 2005-26390, a timing analysis is performed using layout data. Then, it is judged whether a delay time of a signal fulfills a predetermined reference value. When the predetermined reference value is not fulfilled, a redundant via is added for the number of via cuts to be increased so that the reference value may be fulfilled.
According to a technique described in Japanese Patent Laid-Open No. 2006-135152, an error analysis is performed for layout data to which a redundant via is added, and presence of an antenna effect error or a timing constraint violation is judged. When an error exists, redundant via structure is restored to a single-cut via so that the error may be dissolved.
According to a technique described in Japanese Patent Laid-Open No. 2006-65403, in order to enhance a substitutional rate to redundant via structure (multi-cut via), formation of a redundant via is performed during a wiring design. Specifically, wiring cost, via cost, and obstruction cost are defined, and a wiring route that cost becomes at minimum is searched. Then, wiring is laid out and single-cut vias are placed (refer to FIG. 8 in the document concerned). Furthermore, the single-cut vias are substituted by multi-cut vias (refer to FIG. 9 in the document concerned).
As a result of investigating the above-mentioned conventional technology, the present inventor paid attention to the following point. According to the above-described conventional technology, “formation of a redundant via” is performed, that is, an original single-cut via is converted into redundant via structural after the single-cut via is placed. Here, a case as shown in FIG. 1 is considered. FIG. 1 shows a state that a wiring 1 is laid out with ranging over the wiring layers M1 and M2, and a single-cut via 2 is placed in order to connect the wiring layers M1 and M2. At this time, another wiring is laid out on the wiring layer M1 or M2 around the single-cut via 2. Therefore, since a space for substituting the single-cut via 2 to redundant via structural does not exist, formation of a redundant via is impossible regarding the wiring 1. In this way, according to the conventional technology, it was not possible to achieve the formation of a redundant via depending on a surrounding wiring situation.